1. Field of the Invention
The present invention relates to an X-ray lithography mask, a light exposure apparatus and a light exposure process such as an X-ray lithography exposure apparatus and an X-ray lithography exposure process.
2. Related Background Art
In a conventional X-ray lithography exposure apparatus, after synchrotron radiation light emitted from an X-ray lithography light source is reflected by an optical system including a mirror apparatus, the light is guided into a sealed exposure chamber through an X-ray filter made of a material such as beryllium (Be) and illuminates an X-ray lithography mask (referred to as an X-ray mask hereinafter) arranged in the exposure chamber to transfer a mask pattern formed on an X-ray transmission membrane of the X-ray mask onto a wafer.
The synchrotron radiation light is radiated in the shape of an unfolded fan from the X-ray lithography light source. The light diverges in a circular orbital plane (a horizontal plane) while this light is parallel in a plane perpendicular to the circular orbital plane (a perpendicular plane). Further, the synchrotron radiation light has a roughly uniform intensity profile in the circular orbital plane.
Therefore, the following exposure systems have been adopted for uniformly illuminating an exposure area with a synchrotron radiation light.
(1) Mirror vibration system (a plane mirror)
A plane mirror is disposed parallel to the orbital plane of radiation light, and the mirror is vibrated so that the light having a uniform intensity profile in the orbital plane or a plane parallel to the mirror is scanned in a direction perpendicular to the orbital plane to illuminate an X-ray mask. The control of an exposure amount on the X-ray mask is performed by controlling the vibration speed of the plane mirror.
(2) Mirror vibration system (a curved mirror)
Radiation light is condensed and then collimated using two curved mirrors. The second mirror is vibrated similar to the above system (1) to scan the radiation light over an exposure area on an X-ray mask. The control of an exposure amount is also performed by controlling the vibration speed of the second mirror similarly.
(3) Electron beam rocking system
Electrons are rocked in a sinusoidal function manner at an emission point in an accumulating ring of an X-ray lithography light source so that radiation light has a uniform intensity profile in a direction perpendicular to an orbital plane as well as in the orbital plane. Although an entire exposure area can be exposed at a shot without any intensity irregularity or unevenness if the light is caused to be incident on an X-ray mask as it is without using an optical system, a mirror has been used in recent years in order to eliminate adverse influences of short wavelength components of the radiation light. As a result, the radiation light incident on the X-ray mask has a uniform intensity profile in a direction parallel to a reflection surface of the mirror while the radiation light is uneven in a direction perpendicular to the reflection surface. Therefore, the exposure amount is changed in-the direction perpendicular to the reflection surface of the mirror to uniformly expose the entire X-ray mask.
(4) Mask-wafer stage scanning system.
Radiation light is reflected by a stationary plane mirror horizontally disposed parallel to an orbital plane. Since a reflected light has uniform intensity profile in a plane parallel to the orbital plane or a mirror reflection surface, a mask-wafer stage is scanned in a direction perpendicular to this plane. The control of the exposure amount is performed by controlling the moving speed of the mask-wafer stage.
(5) Stationary convex mirror system
A cylindrical curved mirror is arranged in a direction parallel to an orbital plane of radiation light, and the radiation light is diverged in a direction perpendicular to the orbital plane while being reflected as it is in a direction parallel thereto. Since the light incident on an X-ray mask has a uniform intensity profile in the orbital plane of the radiation light, a shutter having an opening extending parallel to the orbital plane is arranged and moved in that perpendicular direction. As a result, the exposure amount is controlled by the moving speed of the shutter.
These systems are common in the following point. While the light incident on the X-ray mask and the wafer is substantially uniform in the plane parallel to the mirror reflection surface, the tight has an uneven intensity profile in the direction perpendicular to the mirror reflection surface. Therefore, a mirror, shutter or mask-wafer stage is moved, and the control of the exposure amount in the direction perpendicular to the mirror reflection surface is performed by adjusting its moving speed. Thus, the entire exposure area is uniformly illuminated.
The X-ray mask supports an X-ray transmission membrane or film by a supporting frame, and the transmission membrane transmits synchrotron radiation light of an exposure wavelength therethrough. Further, the membrane should be a strained self-sustained membrane having a good transmissivity to an alignment light (visible light, infrared light). In general, an inorganic or organic membrane having a thickness from 1 .mu.m to 6 .mu.m is used as such a membrane. The organic membrane, however, is disadvantageous in X-ray resistance, dimensional stability and the like, and therefore, the inorganic membrane is presently being dominant.
Boron nitride, silicon, silicon nitride, silicon carbide, etc., are mainly known as inorganic materials of such an X-ray transmission membrane. Among them, silicon nitride is most widely used since appropriate values of X-ray, visible and infrared light transmission factors, strength, strain characteristic, X-ray resistance and the like can be obtained and its stable manufacturing is possible.
The X-ray transmission membrane of inorganic material is produced as follows: In general, a membrane of a desired material consisting of a single layer or multi-layers is formed on a substrate of silicon or tile like by a vapor deposition method, etc., and-thereafter a structure of an X-ray absorber or the like is formed thereon when necessary. The substrate in an area for transmitting synchrotron radiation light therethrough is then chemically removed from the substrate bottom. At this time, the control of internal stress at the time of a membrane fabrication by the vapor deposition method is important in order to make a strained self,sustained X-ray transmission membrane and to obtain an X-ray mask having a high dimensional stability. Therefore, chemical vapor deposition (CVD) is frequently used to form the membrane because the CVD is capable of largely changing the stress by altering its fabrication condition.
In the CVD, a volatile chemical compound for making -the membrane is vaporized and fed into a reaction chamber as a reactive gas and a chemical reaction is conducted on the substrate in the reaction chamber using heat, plasma, light or the like so that a thin film is formed on the substrate. The CVD is classified into low pressure chemical vapor deposition and atmospheric pressure chemical vapor deposition depending on the pressure in the reaction chamber.
In connection with an X-ray transmission membrane of the X-ray mask, its thin film or membrane forming mechanism is deeply related to diffusion of a reactive gas onto the substrate surface. Therefore, a roughly one-dimensional film thickness irregularity (a flow pattern) is likely to appear in the formed membrane along a diffusion direction of the reactive gas or a direction in which the reactive gas flows. In the low pressure CVD, the film thickness irregularity is considerably reduced since a mean free path of the reactive gas and its diffusion coefficient are relatively large. Still, the film thickness irregularity on one wafer in the low pressure CVD amounts to about 2 to 3%. In the atmospheric pressure CVD, this uneveness amounts to about 5 to 6%.
Further, a sputtering deposition method, an EB (electron beam) deposition method, a vacuum evaporation method and the like other than the CVD can be used, but film thickness irregularity occurs in these film fabrication methods. This irregularity often becomes a nearly one-dimensional one similar to the above, due to a geometrical disposition-between an evaporation source and a substrate.
In the X-ray lithography apparatus, the intensity profile of radiation light is likely to be deviated from an ideal profile due to undesirable characteristics of various optical elements as well as the above-discussed properties of the synchrotron radiation light and the optical system. As a result, its uneveness occurs.
Conventionally, the ideal intensity profile has been corrected by various exposure systems in order to uniformly illuminate the exposure area as discussed above. However, the irregularity deviated from the ideal intensity profile was not corrected. Thus, the irregularity of the exposure amount occurs on the wafer.
Reflection factor irregularity and profile error of an optical system mirror, film thickness irregularity of an X-ray filter, posture setting error, adhesion of contaminants to an optical element, etc., can be enumerated as reasons for causing the exposure amount irregularity. The film thickness irregularity of the X-ray transmission membrane is also a reason for a large irregularity of an exposure amount. Conventionally, only an effort to increase accuracies of various optical elements and systems has exclusively been made in order to reduce the exposure amount or light irregularity.
The exposure amount irregularity in the X-ray lithography appears as a line width error of a transfer pattern as mentioned above. For example, it is reported that a tolerance of the exposure amount fluctuation should be within .+-.20% when a gate layer pattern of 0.2 .mu.m is to be formed, in order to restrain the fluctuation of the transfer pattern width of the gate layer pattern within .+-.10% (see K. Deguchi et. al. NTT R&D 39 601 (1990)). Actually, the line width error needs to be limited within .+-.2.5%, and the exposure amount irregularity should be restricted within 5% (this is a maximum tolerance range) since reasons for the line width error involve dimensional errors and the like at the time of an X-ray mask fabrication other than the exposure amount irregularity.
The exposure light irregularity of about 2% is caused from the above-discussed reflection factor irregularity and profile error of the optical system mirror, while that of about 1% results from the film thickness irregularity of the X-ray filter. Considering other exposure amount irregularities, it is desirable that the exposure amount irregularity due to the film thickness irregularity of-the X-ray transmission membrane should be within 1%.
The magnitude of the film thickness irregularity that causes the exposure light irregularity of 1% differs depending on the material of the X-ray transmission membrane and the wavelength of the used x-ray used. For example, in a case when the synchrotron radiation light is reflected by a silicon carbide mirror and the light having a center wavelength of 1 .mu.m obtained after being passed through a beryllium filter is used, the film thickness irregularity that causes the exposure light irregularity of 1% for the X-ray transmission membrane of silicon nitride having a film thickness of 2 .mu.m is about 34 nm. This value corresponds to 1.7% of the film thickness.
Therefore, the film thickness irregularity of the X-ray transmission membrane should be below 34 nm (1.7%) in such a system in order to make the exposure light irregularity due to the film thickness irregularity of the X-ray transmission membrane smaller than 1%.
In the membrane fabricated by the above-discussed CVD, however, the film thickness irregularity of at least more than 2 to 3% occurs. Thus, this results in the exposure light irregularity of more than 1% due to the film thickness irregularity.
In the conventional X-ray lithography exposure apparatus, however, there is not provided any means for correcting the exposure light irregularity due to the film thickness irregularity.